JP4728129B2 - Marine steel with excellent corrosion resistance and toughness - Google Patents

Description

  The present invention relates to marine steel materials having excellent corrosion resistance and toughness, and more particularly to marine steel materials having excellent corrosion resistance and toughness in an environment exposed to salt and constant temperature and humidity in seawater and in environments containing sulfur. . The steel material of the present invention is suitably used for main structural materials (outer plates, ballast tanks, crude oil tanks, etc.) in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships and warships.

  Major structural materials in ships such as tankers that transport petroleum-based fuels are exposed to constant temperature and high humidity containing salt, or to environments containing sulfur derived from petroleum and sulfates in seawater. Therefore, it is required to have excellent corrosion resistance. For this reason, until now, paints have been applied to marine steels and anticorrosion has been carried out.

  Of these, when coatings represented by heavy coating are performed, there is a high possibility that a coating film defect is generated, and the coating film may be damaged due to a collision or the like in the manufacturing process, so the base steel material is often exposed. . In such a steel exposed portion, since local corrosion (local corrosion) proceeds intensively, for example, petroleum-based liquid fuel stored in a petroleum tank leaks early.

  On the other hand, although the anticorrosion method is very effective for a part completely immersed in seawater, an electrical circuit necessary for anticorrosion is not formed in a part that receives seawater splashes in the atmosphere. The anticorrosion effect may not be fully exhibited. In addition, if the anticorrosive galvanic anode is consumed or lost due to dropping off, severe corrosion may proceed immediately.

  In addition to the above, techniques for improving the corrosion resistance of the steel material itself have also been proposed (Patent Document 1, Patent Document 2, etc.). Patent Document 1 describes a technique for corrosion-resistant steel for shipbuilding in which the contents of Mg and Cu are appropriately controlled and has excellent corrosion resistance even without coating. Further, Patent Document 2 relates to a technology for marine steel materials in which the contents of Ni and Cu are appropriately controlled, and exhibits excellent coating damage resistance even when applied to a ballast tank exposed to a severe corrosive environment. It is described to do.

  However, any of the above methods is inferior in the effect of improving the corrosion resistance under a more severe corrosive environment. In particular, the corrosion resistance against local corrosion is insufficient, and in particular, the deterioration of the corrosion resistance against crevice corrosion poses a serious problem. Crevice corrosion is corrosion that occurs at the `` crevice '' part such as the contact part between steel and foreign matter or the damaged part of the anticorrosion coating film, and because of the high corrosion rate, it is the main cause of shortening the life of ships etc. Yes.

  Such local corrosion and crevice corrosion are particularly noticeable in petroleum tanks such as petroleum liquid fuel tanks. In the case of petroleum tanks, local corrosion proceeds remarkably at the defective part of the oil coat formed on the steel plate surface. This defective part is thought to be repaired or newly formed by the movement of crude oil, deformation of the hull, etc. during operations of crude oil tankers, etc., so local corrosion does not concentrate in one place, Progresses to almost the entire surface of steel. In addition, since crevice corrosion also occurs remarkably in petroleum tanks, it is desired to improve steel materials having excellent local corrosion resistance and crevice corrosion resistance.

As a technique for improving corrosion resistance against local corrosion, for example, Patent Document 3 to Patent Document 5 can be cited. These all relate to a technique in which chemical components in steel are appropriately controlled. For example, Patent Document 3 discloses a crude oil in which the contents of Cu, Ni, Cr, Mo, Sb, and Sn are appropriately controlled. Corrosion resistant steel for heavy oil storage, Patent Document 4 discloses a steel material for a cargo oil tank in which the contents of Cu, Ni, Cr, and Al are appropriately controlled. Patent Document 5 discloses Cu, Ni, Mo, and Cr. Steel materials for crude oil tank bottom plates whose contents are appropriately controlled are described respectively. Although these techniques increase the corrosion resistance against general corrosion and local corrosion, further improvement is desired. In addition, none of the above-mentioned patent documents pays sufficient attention to the corrosion resistance against crevice corrosion. Therefore, improvement of crevice corrosion resistance is strongly desired.
JP, 2000-17381, A Claims etc. JP, 2002-266052, A Claims etc. JP, 2001-214236, A Claims etc. JP, 2003-82435, A Claims etc. JP, 2004-2948, A Claims etc.

  As described above, in the marine steel materials, in particular, although the improvement of the corrosion resistance against crevice corrosion is strongly desired, the method described in the above-mentioned patent document does not sufficiently satisfy the required characteristics.

  Furthermore, marine steel materials are also required to have excellent toughness in consideration of ensuring safety during a collision.

  This invention is made | formed in view of the said situation, The objective is to provide marine steel materials excellent in corrosion resistance and toughness. Specifically, even if it does not give paint or cathodic protection, it contains salt. An object of the present invention is to provide a marine steel material that has improved corrosion resistance in a constant temperature and high humidity environment or an environment containing sulfur, and in particular, the corrosion resistance against crevice corrosion is remarkably enhanced.

The marine steel material of the present invention that has solved the above-mentioned problems is: C: 0.01 to 0.30% (meaning mass%; the same applies hereinafter), Si: 0.01 to 2.0%, Mn: 0 0.01 to 2.0%, Al: 0.005 to 0.10%, Bi: 0.0005 to 0.40%, P: 0.003 to 0.050%, balance: Fe and inevitable impurities And there is a gist in satisfying the following formulas (1) and (2).
[P] × 7 + [Bi] <0.50% (1)
0.050 ≦ [P] / [Bi] ≦ 5.0 (2)

  In a preferred embodiment, Cu: 0.01-5.0%, Ni: 0.01-5.0%, Cr: 0.01-5.0%, and Ti: 0.005-0.20 % At least one selected from the group consisting of%.

  In preferable embodiment, Ca: 0.0005-0.020% and / or Mg: 0.0005-0.020% are contained further.

  In a preferred embodiment, it further contains at least one selected from the group consisting of Sn: 0.001 to 0.30%, As: 0.001 to 0.30%, and Sb: 0.001 to 0.30%. To do.

  In a preferred embodiment, it further contains at least one selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50%. To do.

  In a preferred embodiment, the steel material is used for a crude oil tank.

  Since the present invention has the above-described configuration, it provides a marine steel material that has improved overall corrosion resistance, corrosion uniformity, crevice corrosion resistance, and paint corrosion resistance, and is excellent in toughness. I was able to. The steel material of the present invention is suitably used for structural materials (outer plates, ballast tanks, crude oil tanks, etc.) in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships, etc., and particularly suitable for crude oil tanks. It is done.

  The present inventor has improved corrosion resistance especially against crevice corrosion occurring in the crevice, among corrosion due to salt adhesion and wetting caused by seawater, and corrosion caused by sulfur from petroleum and sulfates in seawater. In order to provide the obtained steel material, it examined. As a result, when Bi is positively added and Bi is controlled within a predetermined range in relation to P, it is found that a desired corrosion resistance is ensured and a steel material excellent in toughness can be obtained. completed.

  Thus, although the mechanism in which corrosion resistance improves by controlling Bi and P is unknown in detail, it is guessed as follows.

First, we consider the mechanism of corrosion in ships exposed to seawater and constant temperature and humidity. In the above corrosive environment, Fe ²⁺ ions generated by dissolution of steel materials used in ships are hydrolyzed and the pH is lowered to become acidic, so the main cathode reaction (reduction reaction) It is thought to be a reaction between the sulfur content derived from sulfate and the like and hydrogen ions. On the other hand, in the case of containers such as petroleum tanks that come into contact with petroleum-based fuels, the main cathode reaction is considered to be a reaction between sulfur content derived from petroleum-based fuels and hydrogen ions. In any environment, it is considered that the cathodic reaction between sulfur and hydrogen ions is mainly involved. Under such a corrosive environment, when Bi and P are added in a predetermined range, the reaction from the steel material Phosphate and bismuth produced by elution act on the site where the cathodic reaction of corrosion occurs (cathode site), and remarkably suppress the cathodic reaction. Such a suppressive action is particularly prominent against the corrosion of the crevice portion under the coating film, so that it is presumed that the crevice corrosion resistance is improved and the coating corrosion resistance is also improved.

  In order to effectively exhibit the above action, Bi and P are added as follows, and the content [P] of P and the content [Bi] of Bi satisfy the following formulas (1) and (2): It is necessary to be satisfied.

Bi: 0.0005 to 0.40%
Bi produces bismuthate in a corrosive environment in which the cathode reaction between sulfur and hydrogen ions is mainly involved as described above, and bismuthate acts on the cathode site and oxidizes. It is an element that contributes to improving corrosion resistance. In order to effectively exhibit such an action, Bi is added in an amount of 0.0005% or more. However, if Bi is added excessively, Bi will segregate at the grain boundaries to cause deterioration of toughness, and weldability will decrease, so the upper limit is made 0.40%. The addition amount of Bi is preferably 0.0008% or more and 0.38% or less, and more preferably 0.010% or more and 0.35% or less.

P: 0.003 to 0.050%
In the corrosive environment in which the cathode reaction between sulfur and hydrogen ions is mainly involved as described above, P generates phosphate and adsorbs on the cathode site to improve corrosion resistance. It is a contributing element. In order to exhibit such an action effectively, 0.003% or more of P is added. However, if P is added excessively, the toughness and weldability are deteriorated, so the upper limit is made 0.050%. The addition amount of P is preferably 0.045% or less, and more preferably 0.040% or less.

[P] × 7 + [Bi] <0.50% (1)
As described above, both Bi and P are elements that contribute to the improvement of corrosion resistance. However, when added excessively, they adversely affect toughness and weldability, and the degree of adverse effects due to P is more than 7 times that of Bi. Something has been clarified by experiments of the present inventors. As a result of further investigation based on this knowledge, it has been found that it is effective to satisfy the range of the above formula (1) in order to prevent adverse effects due to Bi and P (see the examples described later). ). The above formula (1) is preferably less than 0.45%, more preferably 0.40% or less.

  The lower limit of the above formula (1) can be inevitably determined by the lower limits of [P] and [Bi] defined in the present invention, but is generally preferably 0.02%, preferably 0.04%. It is more preferable that

0.050 ≦ [P] / [Bi] ≦ 5.0 (2)
It is considered that the corrosion resistance improving action according to the present invention is exhibited mainly by the synergistic action of the adsorption action of phosphate produced by dissolution of P and the oxidation action of bismuth acid salt produced by dissolution of Bi. However, it is the present inventor that such an action is effectively exhibited not only by controlling the contents of P and Bi individually but also by appropriately controlling the ratio of these contents. It became clear by experiment. As shown in the examples to be described later, when the range of the above formula (2) is not satisfied, the corrosion uniformity and crevice corrosion resistance decrease. The ratio of [P] / [Bi] is preferably 0.055 or more and 4.5 or less, and more preferably 0.06 or more and 4.0 or less.

  The components that characterize the present invention have been described above.

  Next, other steel components used in the steel material of the present invention will be described.

C: 0.01 to 0.30%
C is an element necessary for ensuring the strength of the material. C is added in an amount of 0.01% or more in order to ensure the minimum level of strength required for the structural members of the ship (approximately 400 MPa, depending on the thickness of the steel used). However, if it exceeds 0.30% and is added excessively, toughness deteriorates. The C content is preferably 0.02% or more and 0.28% or less, and more preferably 0.04% or more and 0.26% or less.

Si: 0.01 to 2.0%
Si is added for deoxidation and ensuring strength. If Si is less than 0.01%, the minimum level of strength required for the construction of ship structural members cannot be ensured. However, when Si exceeds 2.0% and becomes excessive, weldability deteriorates. The content of Si is preferably 0.02% or more and 1.5%, and more preferably 0.05% or more and 1.0% or less.

Mn: 0.01 to 2.0%
Mn is added for deoxidation and securing of strength, similar to Si. If Mn is less than 0.01%, the minimum level of strength required for the structural member cannot be ensured. However, when Mn exceeds 2.0% and becomes excessive, toughness deteriorates. The Mn content is preferably 0.05% or more and 1.80%, and more preferably 0.10% or more and 1.60% or less.

Al: 0.005-0.10%
Al, like Si and Mn, is added for deoxidation and securing of strength. If Al is less than 0.005%, the deoxidizing action cannot be exhibited effectively. However, if Al is added in excess of 0.10%, the weldability decreases. The Al content is preferably 0.008% or more and 0.090%, and more preferably 0.010% or more and 0.080% or less.

  The steel material of the present invention contains the above components, and the balance is iron and inevitable impurities (for example, S, N, O, etc.).

  It should be noted that other permissible components (for example, Zr, Hf, etc.) may be further added to such an extent that the action of the present invention is not inhibited. Since these tolerable components are deteriorated in toughness when the content thereof is excessive, it is preferable to suppress them to about 0.1% or less in total.

  In the present invention, if necessary, (i) at least one selected from the group consisting of Cu, Ni, Cr, and Ti, (ii) a group consisting of Ca and / or Mg, (iii) Sn, As, and Sb (Iv) At least one selected from the group consisting of B, V, and Nb may be further added, and the properties of the steel material are further improved according to the type of the additive component.

(I) The group consisting of Cu: 0.01 to 5.0%, Ni: 0.01 to 5.0%, Cr: 0.01 to 5.0%, and Ti: 0.005 to 0.20% Any one of these elements selected from is effective for improving corrosion resistance.

  Of these, Cu, Cr, and Ni greatly contribute to improving corrosion resistance by forming a dense rust film on the surface. In particular, Ni is an element that contributes to the improvement of coating corrosion resistance by suppressing corrosion under the coating film. In order to effectively exhibit such an action, it is preferable to add any element in an amount of 0.01% or more. However, if excessively added, any of the elements deteriorates weldability and hot workability. The element content is also preferably 5.0% or less. Cu, Cr, and Ni are each more preferably 0.05% or more and 4.50% or less.

  On the other hand, Ti is an element that, in addition to the above-described effects, suppresses corrosion of the coating film buttock and improves coating corrosion resistance. However, when it adds excessively, weldability and workability will deteriorate. The Ti content is more preferably 0.008% or more and 0.15% or less.

  These elements may be added alone or in combination of two or more.

(Ii) Ca: 0.0005 to 0.020% and / or Mg: 0.0005 to 0.020%
Ca and Mg exhibit an action of increasing pH when dissolved, suppress a decrease in pH due to a hydrolysis reaction of a local anode where iron is dissolved, suppress a corrosion reaction, and improve corrosion resistance. Such an effect is effectively exhibited by adding 0.0005% or more of Ca and Mg. However, when Ca and Mg are added excessively exceeding 0.020%, workability and weldability are deteriorated. The contents of Ca and Mg are more preferably 0.0010% or more and 0.015% or less, respectively. Ca and Mg may be added alone or both may be added.

(Iii) At least one of these elements selected from the group consisting of Sn: 0.001 to 0.30%, As: 0.001 to 0.30%, and Sb: 0.001 to 0.30% , The rust densification action by the element (i) (Cu, etc.) is promoted to further improve the corrosion resistance. In order to effectively exhibit such an action, it is preferable to add 0.001% or more of any of the above elements. However, if these elements are added excessively, the workability and weldability deteriorate, and therefore, it is preferable that both elements be 0.30% or less. The content of the elements is more preferably 0.005% or more and 0.25% or less. These elements may be added alone or in combination of two or more.

(Iv) At least one of these elements selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50% , It contributes to the improvement. However, if these elements are added excessively, the base material toughness decreases. Taking these into consideration, the preferable content of the above elements is set in the above range. The content of B is more preferably 0.0003% or more and 0.0090% or less. The content of V is more preferably 0.02% or more and 0.45%. The Nb content is more preferably 0.005% or more and 0.45% or less.

  The steel material of this embodiment may be further coated as needed. For example, as shown in the examples described later, a coating film may be formed using a tar epoxy resin paint. Or you may perform the typical heavy-duty anticorrosion coating methods other than the said coating, for example, the coating by a zinc rich paint and a shop primer. Furthermore, it is possible to use an anticorrosion method such as cathodic protection in combination. By these methods, the corrosion resistance of coating is further improved (see Examples described later).

  The steel material of the present invention not only exhibits excellent corrosion resistance against salt adhesion due to seawater or the like and corrosion due to a wet environment, but also when applied to, for example, a petroleum liquid fuel tank, the corrosive environment Excellent corrosion resistance can be exhibited below.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and may be implemented with appropriate modifications within a range that can meet the gist of the preceding and following description of the present specification. Included in the technical scope.

Example 1
In this example, the test piece A to the test piece C manufactured as follows is used, and a corrosion test simulating an environment in which a ship is corroded (combined cycle corrosion by repeated seawater spray test and constant temperature and humidity test). Test). The detailed experimental method is as follows.

(Preparation of test piece A)
First, steel materials (base materials) No. 1 having various component compositions (remainder: Fe and inevitable impurities) shown in Tables 1 and 2 are used. 1 to 35 were melted in a converter and various steel plates were produced by continuous casting and hot rolling. The obtained steel sheet was cut and the surface was ground to prepare a test piece A (size: about 100 mm × about 100 mm × about 25 mm) shown in FIG.

(Preparation of test piece B for crevice corrosion resistance measurement)
The test piece B shown in FIG. 2 was produced as follows.

  First, as shown in FIG. 2, four small test pieces for forming a gap of about 20 mm × about 20 mm × about 5 mm are brought into contact with a large test piece having the same size as the above-mentioned test piece A to form a gap portion. A test piece B was prepared. Here, the compositions of the small test piece for forming the gap and the large test piece are all the same. Further, the surface of the small test piece for forming a gap was ground in the same manner as the large test piece. As shown in FIG. 2, a hole of about 5 mmφ was provided at the center of the small test piece for forming a gap, and a screw hole (not shown) was opened on the substrate side (large test piece side) and fixed with an M4 plastic screw.

(Preparation of test specimen C for coating corrosion resistance measurement)
In order to examine the coating corrosion resistance when the steel material to which the anticorrosion coating film was applied was damaged and the base steel material was exposed, a test piece C shown in FIG. 3 was prepared as follows.

  First, a zinc rich primer undercoating (average thickness of about 15 μm) and a modified epoxy resin (average thickness) were applied to the entire surface of a steel plate (size of about 100 mm × about 100 mm × about 25 mm) prepared in the same manner as the test piece A described above. About 250 μm) was applied in order. Next, using a cutter knife on one side of this test piece, as shown in FIG. 3, a cut wound (length of about 100 mm, width of about 0.5 mm) reaching the substrate is formed, and the test piece for measuring the coating corrosion resistance C was obtained.

(Measurement method for corrosion test)
The following corrosion tests were performed on the specimen A, the crevice corrosion resistance measurement specimen B, and the coating corrosion resistance measurement specimen C thus obtained. This corrosion test simulates the environment in which a ship is corroded, and as shown below, a combined cycle corrosion test was performed by repeating a seawater spray test and a constant temperature and humidity test.

First, a seawater spray test is performed for 4 hours. Specifically, the test pieces A to C were installed in a test tank in a state where the test pieces A to C were inclined at an angle of 60 ° from the horizontal, and 35 ° C. artificial seawater (salt water) was sprayed in a mist form. The spraying of salt water was continuously performed continuously for a total of 4 hours. In the test tank, a circular dish with an area of 80 cm ² is installed horizontally, and the spray amount is 1.5 ± 0.3 mL of artificial seawater per hour so that it can be collected at any position on this circular dish. Is adjusted in advance.

  Next, a constant temperature and humidity test is performed for 4 hours. Specifically, the test pieces A to C were placed in a constant temperature and humidity test tank adjusted to a temperature of 60 ° C. and a humidity of 95% in a state inclined at an angle of 60 ° from the horizontal.

  In the combined cycle corrosion test, the seawater spray test was performed for 4 hours and the constant temperature and humidity test was performed for 4 hours, and this was repeated for a total of 6 months.

(Corrosiveness of specimen A)
The corrosivity (total corrosion resistance and corrosion uniformity) of the test piece A was evaluated as follows.

  First, in order to remove corrosion products such as iron rust formed on the test piece A, cathodic electrolysis was performed in a diammonium hydrogen citrate aqueous solution based on JIS K8284.

  Next, the mass of the test piece A before and after the corrosion test was measured and converted into an average reduction amount D-ave (mm) of the plate thickness. The average reduction amount D-ave of five test pieces was calculated in the same manner, and the overall corrosion resistance was evaluated.

  Moreover, the maximum erosion depth D-max (mm) of the test piece A was calculated | required using the stylus type three-dimensional shape measuring apparatus. The maximum erosion depth D-max (D-max / D-ave) with respect to the average reduction amount D-ave of the plate thickness calculated as described above was calculated, and the corrosion uniformity was evaluated.

(Crevice corrosion resistance of specimen B)
The crevice corrosion resistance was evaluated by measuring the maximum crevice corrosion depth D-crev (mm) as follows.

  First, after removing the crevice forming test piece attached to the test piece B, the crevice portion (contact surface) was visually observed, and in the same manner as the test piece A, iron rust or the like was observed for crevice corrosion. Corrosion products were removed.

  Next, the maximum crevice corrosion depth D-crev (mm) on the large test piece side was measured using the stylus type three-dimensional shape measuring apparatus used for the evaluation of corrosivity.

(Coating corrosion resistance of test piece C)
The coating corrosion resistance was evaluated by measuring the maximum swollen width as follows.

  First, five test pieces C were prepared, and the swollen width (mm) of the coating film in the direction perpendicular to the cut flaw was measured with a caliper, and the maximum value of each measured value was defined as “maximum swollen width”.

  Table 3 shows the evaluation criteria of the corrosion resistance (total corrosion resistance and corrosion uniformity) of the test piece A, the crevice corrosion resistance of the test piece B, and the coating corrosion resistance (maximum swollen width) of the test piece C. In Table 3, when each evaluation item of corrosion resistance was “◯” or “◎”, it was defined that the corrosion resistance was excellent (accepted), and the overall judgment was “◯” or “◎”. On the other hand, those having at least one “Δ” in each evaluation item of corrosion resistance were defined as inferior in corrosion resistance (failed).

In this example, in addition to the corrosion test, a Charpy impact test was performed at 0 ° C. to measure the absorbed energy (vE ₀ ), and the HAZ toughness was evaluated. Specifically, a steel sheet having a thickness of 30 mm was used, and after welding (electrogas arc welding) at a heat input of 60 kJ / mm, a JIS No. 4 test piece was collected so that a 1/4 part of the thickness was the center line ( 7), a Charpy impact test was performed at 0 ° C., and the absorbed energy (vE ₀ ) of the weld fusion line (bond portion) at a ¼ portion of the plate thickness was obtained. When vE ₀ was vE ₀ ≧ 100 J, the pass (O) was accepted, and vE ₀ <100 J was rejected (X).

  In this example, a sample excellent in both corrosion resistance and HAZ toughness was used as an example of the present invention, and a sample inferior to at least one of corrosion resistance and HAZ toughness was used as a comparative example.

  Tables 4 and 5 show the results of evaluation according to the corrosive evaluation criteria shown in Table 3, and the results of toughness.

  From Table 4 and Table 5, it is considered as follows.

  First, No. 4 in Table 4 was used. 6-20, No. 5 in Table 5. 21 to 35 are examples of the present invention that satisfy the requirements of the present invention, and are excellent in corrosion resistance compared to conventional steel (No. 1 in Table 4) and the like. Among these, No. 1 further contains an element for improving corrosion resistance such as Cu, Cr, Ni, Ti. 10-35 are much more excellent in corrosion resistance, and it turns out that it is very useful as a steel material for ships.

  In contrast, conventional steel containing no Bi (No. 1 in Table 4) is inferior in corrosion resistance.

  No. No. 2 is a comparative example with a low Bi content. 4 and no. 5 is a comparative example in which the content ratio of P and Bi ([P] / [Bi]) is below or exceeds the range of the present invention. These comparative examples are Nos. Of conventional steels. Compared to 1, the overall corrosion resistance was slightly improved, but no improvement in corrosion uniformity or crevice corrosion resistance was observed.

On the other hand, no. 3 is a comparative example in which the Bi content and [P] / [Bi] satisfy the requirements of the present invention, but the value of [P] × 7 + [Bi] exceeds the range of the present invention. No. of steel Although corrosion resistance was improved compared to 1, vE ₀ was less than 100 J, which is inferior to HAZ toughness.

Example 2
In this example, instead of the corrosion test simulating the seawater environment as in Example 1, the ship was actually operated to conduct the corrosion test (practical exposure test). Specifically, the corrosion state and toughness when the test piece D to the test piece F produced as follows were attached to a crude oil tanker and operated were evaluated. The size (excluding thickness) of the test pieces D to F used here is three times that of the test pieces A to C used in Example 1, and in the same manner as in Example 1, the overall corrosion resistance and corrosion Uniformity, crevice corrosion resistance, and paint corrosion resistance were measured to evaluate the corrosion resistance.

  The detailed experimental method is as follows.

(Preparation of test piece D)
First, steel materials (base materials) No. 1 having various component compositions (remainder: Fe and inevitable impurities) shown in Tables 1 and 2 described above. 1 to 35 were melted in a converter and various steel plates were produced by continuous casting and hot rolling. The obtained steel plate was cut and the surface was ground to prepare a test piece D (size: about 300 mm × about 300 mm × about 25 mm) shown in FIG.

(Preparation of test piece E for crevice corrosion resistance measurement)
The test piece E shown in FIG. 5 was produced as follows.

  First, as shown in FIG. 5, four small test pieces for forming a gap of about 60 mm × about 60 mm × about 5 mm are brought into contact with a large test piece having the same size as the test piece D described above to form a gap portion. A test piece E was prepared. Here, the compositions of the small test piece for forming the gap and the large test piece are all the same. Further, the surface of the small test piece for forming a gap was ground in the same manner as the large test piece. As shown in FIG. 5, a hole of about 10 mmφ was provided at the center of the small test piece for forming a gap, and a screw hole (not shown) was opened on the substrate side (large test piece side) and fixed with an M8 plastic screw.

(Preparation of test specimen F for coating corrosion resistance measurement)
In order to investigate the coating corrosion resistance when the steel material to which the anticorrosion coating film was applied was damaged and the base steel material was exposed, a test piece F shown in FIG. 6 was prepared as follows.

  First, a zinc rich primer undercoat (average thickness of about 250 μm) and a modified epoxy resin (average thickness) were applied to the entire surface of a steel plate (size of about 300 mm × about 300 mm × about 25 mm) prepared in the same manner as the test piece D described above. About 250 μm) was applied in order. Next, using a cutter knife on one side of this test piece, as shown in FIG. 6, a cut wound (about 300 mm in length and about 0.5 mm in width) reaching the substrate is formed, and the test piece for coating corrosion resistance measurement is formed. F was obtained.

(Measurement method for corrosion test)
The following corrosion tests were performed on the specimen D, the crevice corrosion resistance measurement specimen E, and the paint corrosion resistance measurement specimen F thus obtained. As described above, this corrosion test is set assuming an environment in which a ship is corroded.

  First, 10 pieces of each of the above test pieces were attached to the bottom plate of the tank inner surface of the VLCC (Very Large Crude Carrier) crude oil tanker and the back of the upper deck, and after normal operation for 5 years, the corrosion of each test piece was observed. The degree of was examined as follows.

(Corrosiveness of specimen D (overall corrosion resistance and corrosion uniformity))
For the overall corrosion resistance, the average reduction amount D-ave (mm) of the plate thickness is calculated in the same manner as the above-described method for measuring the corrosion resistance of the test piece A, and the average reduction amount D-ave of 10 test pieces is calculated. It was evaluated by.

  For the corrosion uniformity, the maximum erosion depth D-max (D-max / D-ave) with respect to the average reduction D-ave in the plate thickness is calculated in the same manner as the above-described method for measuring the corrosivity of the test piece A. It was evaluated by.

(Crevice corrosion resistance of specimen E)
For crevice corrosion resistance, the maximum crevice corrosion depth D-crev (mm) on the large test piece side of each test piece was measured in the same manner as in the above-described method for measuring the corrosivity of test piece B, and the maximum value of each measured value. Was defined as “maximum crevice corrosion depth D-crev (mm)”.

(Coating corrosion resistance of test piece F)
For coating corrosion resistance, the coating film swollen width (mm) of each test piece was measured in the same manner as the above-described method for measuring the corrosiveness of the test piece C, and the maximum value of each measured value was defined as “maximum swollen width”.

  Table 6 shows the evaluation criteria of the corrosion resistance (overall corrosion resistance and corrosion uniformity) of the test piece D, the crevice corrosion resistance of the test piece E, and the coating corrosion resistance (maximum swollen width) of the test piece F. In Table 6, when each evaluation item of corrosion resistance was “◯” or “◎”, it was defined that the corrosion resistance was excellent (accepted), and the overall judgment of corrosion resistance was “◯” or “◎”. On the other hand, those having at least one “Δ” in each evaluation item of corrosion resistance were defined as inferior in corrosion resistance (failed).

  In this example, in addition to the corrosion test, the HAZ toughness was evaluated in the same manner as in Example 1 described above.

  Here, a sample excellent in both corrosion resistance and HAZ toughness was used as an example of the present invention, and a sample inferior to at least one of corrosion resistance and HAZ toughness was used as a comparative example.

  Tables 7 and 8 show the results of evaluation according to the corrosive evaluation criteria shown in Table 6 and the results of toughness.

  The experimental results shown in Table 7 and Table 8 are exactly the same as the experimental results shown in Table 4 and Table 5 in Example 1 described above. That is, when steel materials satisfying the requirements of the present invention are used, No. in Table 7 6-20, No. 8 in Table 8. As shown in Tables 21 to 35, when steel materials that are excellent in both corrosion resistance and toughness but do not satisfy the requirements of the present invention are used, As shown to 1-5, both the corrosion resistance and toughness fell, or the malfunction that corrosion resistance was favorable but toughness fell was seen.

  From the experimental results of this example, it was proved that excellent corrosion resistance and toughness were ensured even in a practical corrosion environment if a steel material satisfying the requirements of the present invention was used.

FIG. 1 is an explanatory view showing the external shape of the test piece A used in Example 1. FIG. FIG. 2 is an explanatory view showing the appearance of the crevice corrosion resistance test piece B used in Example 1. FIG. FIG. 3 is an explanatory view showing the appearance of the test piece C for coating corrosion resistance measurement used in Example 1. FIG. FIG. 4 is an explanatory view showing the external shape of the test piece D used in Example 2. FIG. FIG. 5 is an explanatory view showing the external shape of the test piece E for crevice corrosion resistance measurement used in Example 2. FIG. FIG. 6 is an explanatory view showing the external shape of the test piece F for coating corrosion resistance measurement used in Example 2. FIG. 7 is a diagram showing a HAZ toughness specimen collection position.

Claims (5)

C: 0.01 to 0.30% (meaning mass%; the same applies hereinafter), Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, Al: 0.005 to 0. 10%, Bi: 0.0005 to 0.40%, P: 0.003 to 0.050%, balance: Fe and inevitable impurities, satisfying the following formulas (1) and (2) , A marine steel with excellent corrosion resistance and toughness, characterized by being used in crude oil tanks .
[P] × 7 + [Bi] <0.50% (1)
0.050 ≦ [P] / [Bi] ≦ 5.0 (2)
In the formula, [] means the content (%) of each element.   Furthermore, Cu: 0.01 to 5.0%, Ni: 0.01 to 5.0%, Cr: 0.01 to 5.0%, and Ti: 0.005 to 0.20% The steel material according to claim 1 containing at least one selected.   Furthermore, the steel materials of Claim 1 or 2 containing Ca: 0.0005-0.020% and / or Mg: 0.0005-0.020%.   Furthermore, it contains at least one selected from the group consisting of Sn: 0.001 to 0.30%, As: 0.001 to 0.30%, and Sb: 0.001 to 0.30%. 4. The steel material according to any one of 3.   Furthermore, it contains at least one selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50%. 4. The steel material according to any one of 4 above.